Method and system for ring protection switching

ABSTRACT

The present invention provides a protection switching method of a node having communication failure in a ring, in a communication network. In one embodiment, this is accomplished by checking for loss of signal (LOS) at the non-ERPS device or at least one of the node in a communication channel, checking periodically for Continuity Check Message (CCM) at both the node, wherein the CCMs are periodically transmitted from both the node through at least one non-ERPS device, appropriately setting a Remote Destination Indication (RDI) bit in the Continuity Check Message (CCM) generated from at least one node and transmits the same via the non-ERPS device or through the non-ERPS device or through a disjoint communication channel, and blocking port of the node towards the non-ERPS device upon receipt of the fault notification and transmitting signal fail notification message on both the ports in the ring.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/IN2010/000571, filed 30 Aug.2010, which is hereby incorporated by reference. This application claimspriority from Patent Application No. IN 2157/CHE/2009, filed 7 Sep.2009, which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention is directed to the field of data communicationnetworks. More particularly, the present invention provides a protectionswitching method and system of a node having communication failure in aring, in a communication network.

BACKGROUND OF THE INVENTION

Communication networks can be built in many different networktopologies, such as a ring, mesh, linear, linear chain, tree, and thelike. Conventionally, network protection is provided for at layers oneand below, such as through SONET, SDH, and the like. Typical protectionschemes can include Automatic Protection Switching (APS),Uni-directional Path Switched Rings (UPSRs), two and four fiberBi-directional Line Switched Rings (BLSRs), and the like. Traditionally,networks are typically not protected using ring protection schemes atlayer two (e.g., Ethernet) and above. For example, Ethernet-based accessand aggregation networks are typically designed in a tree structure, andthus lack network protection.

Modern networks, such as access and aggregation networks, are movingtowards Ethernet as the universal medium. Ethernet standards areevolving towards carrier-grade performance, and equipment providerstoday provide various different solutions for Ethernet-based networkprotection. For example, layer one protection can be provided throughEthernet-over-SONET or Ethernet-over-SDH solutions. To increase thebandwidth with the Ethernet-over-SONET and Ethernet-over-SDH leads tohigher cost. At layer two, various solutions have been presented, suchas Rapid Spanning Tree Protocol (RSTP), and Spanning Tree Protocol(STP). The main problems with these type of path protection scheme are:(1) switching time in response to a fault is relatively slow, (2) cannotoffer QoS because of in-deterministic tree; and (3) complex circuitry.

Another form of providing protection switching in a layer-2 network isan Ethernet ring Protection Switching (ERPS) which is formed only ofEthernet bridges as per ITU-T G.8032v1 or v2. This network protectionscheme is only restricted for bridges/switches within the chain or loopof the network. Whenever the network has to expand i.e. for aMetropolitan Area Network (MAN), the protection scheme has to includeone or more aggregation devices for data transfer. The standard ITU-TG.8032 is for Layer 2 Ethernet device. There is no ring protectionswitching for a Non-ERPS device. And hence, connecting an ERPS chain tothe Non-ERPS device will not ensure the ERPS protection of the Non-ERPSdevice.

For the reasons stated above, which will become apparent to thoseskilled in the art upon reading and understanding the specification,there is a need in the art for a protection switching method and systemof a node having communication failure in a ring, in a communicationnetwork.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a protectionswitching method of a node having communication failure in a ring, in acommunication network, wherein the communication failure occurred on atleast one non-ERPS device or node or in a communication channel, themethod including checking for loss of signal (LOS) from the non-ERPS orat-least one of the node in a communication channel, periodicallychecking for Continuity Check Messages (CCMs) at both the nodes, whereinthe CCMs are periodically transmitted and received between each of thenode through at least one non-ERPS device, transparently, appropriatelysetting a Remote Defect Indication (RDI) bit in the Continuity CheckMessage (CCM) generated from at least one node and transmits the samevia the non-ERPS device or through the non-ERPS device or over adisjoint communication channel, and blocking port of the node towardsthe non-ERPS device upon receipt of the fault notification andtransmitting signal fail notification message on both the ports in thering.

In another aspect, the invention includes a 50 ms protection switchingsystem of a node having communication failure in a ring, in acommunication network, wherein the communication failure occur on atleast one non-ERPS device or node or in a communication channel, thesystem includes a plurality of nodes interconnected in a ring topology,at least one non-ERPS device within the ring, and a network channelconnected to each of the nodes via at least one of the non-ERPS devicefor transmitting and receiving Continuity Check Message (CCMs), whereineach node is configured for determining a failure condition within thecommunication channel, appropriately setting a Remote Defect Indicator(RDI) in the CCM and transmitting the same to the other node via thenon-ERPS device or through the non-ERPS device or through a disjointcommunication channel, in response to the failure condition, andblocking the port of the node towards the non-ERPS device upon receiptof the fault notification, and transmitting signal fail notificationmessage on both the ports in the ring.

The methods and system, disclosed herein may be implemented in any meansfor achieving various aspects, and may be executed in a form of amachine-readable medium embodying a set of instructions that, whenexecuted by a machine, cause the machine to perform any of theoperations disclosed herein. Additional advantages and features of thepresent invention will be more apparent from the detailed descriptionand accompanying drawings, which illustrate embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of Ethernet Ring Protected Switching (ERPS)network in the Full Ring mode in normal operation.

FIG. 2 shows an example of ERPS network shown in FIG. 1 in which a faulthas occurred between two of the network nodes in the system.

FIG. 3 shows an example of ERPS network shown in FIGS. 1 and 2 in whicha fault is restored between two of the network nodes in the system.

FIG. 4(a-d) shows different modes of operation of two network nodesshowing the preferred protection mechanism as termination of ERPS onIP/MPLS nodes.

FIG. 5 is a state diagram illustrating operation of the protectionmechanism shown in FIG. 4 in accordance with one embodiment of thepresent invention.

FIG. 6 illustrates one embodiment of a suitable computing environment inwhich certain aspects of the invention illustrated in FIGS. 1-5 may bepracticed.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, various aspects of the present inventionwill be described. However, it will be understood by those skilled inthe art that the present invention may be practiced with only some orall aspects of the present invention. For purposes of explanation,specific numbers, materials and configurations are set forth in order toprovide a thorough understanding of the present invention. However, itwill also be apparent to those skilled in the art that the presentinvention may be practiced without these specific details.

Parts of the description will be presented in terms of operationsperformed by a computer system, using terms such as data, state, link,fault, packet, and the like, consistent with the manner commonlyemployed by those skilled in the art to convey the substance of theirwork to others skilled in the art. As is well understood by thoseskilled in the art, these quantities take the form of electrical,magnetic, or optical signals capable of being stored, transferred,combined, and otherwise manipulated through mechanical and electricalcomponents of the computer system; and the term computer system includesgeneral purpose as well as special purpose data processing machines,routers, bridges, switches, and the like, that are standalone, adjunctor embedded.

Additionally, various operations will be described as multiple discretesteps in turn in a manner that is helpful in understanding the presentinvention. However, the order of description should not be construed asto imply that these operations are necessarily order dependent, inparticular, the order of their presentation.

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the invention. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment.

For the ease understanding of the invention, the IP/MPLS nodes orrouters or non-ERPS (Layer 2) devices are interchangeably used in thebelow description.

The present invention provides a method and system for 50 ms protectionswitching in a layer-2 ring topology network. Specifically, theprotection switching method and system of a node having communicationfailure in a ring, in a communication network, where the communicationfailure occurred on one non-ERPS device or node or in a communicationchannel, in a bridged, layer-2 network. The communication network may beor may include one or more network selected from a group of networksconsisting of a layer-2 networks, a local area network, a metropolitanarea network, a ring network and the like. The non-ERPS device may be ormay include an Internet Protocol/Multiprotocol Label Switching(IP/MPLS), router, Virtual Private Local Area Network Services (VPLS),Hierarchical VPLS (HVPLS), Asynchronous Transfer Mode (ATM) switch,Frame Relay, Ethernet Switch, and Resilient Packet Ring (RPR) switch.The communication channel between the node and the non-ERPS device is auni-directional link or a bi-directional link or both.

FIG. 1 is a block diagram illustrating an Ethernet Ring ProtectionSwitching (ERPS) system. The ERPS system 100 consists of a contentserver 105, nodes (110, 115, 120, 125 and 130), routers (135, 140, 145,150), and a fiber 155.

The ERPS system 100 operates on a ring network. One node on the ringnetwork is designated as the master node. Each node will have a primaryport and a secondary port, both known to be able to send control trafficto the master node. Under normal operation only the primary port on themaster node is used to avoid loops (the secondary port on the masternode is blocked for all non-control traffic).

In operation, the nodes (110, 115, 120, 125 and 130) and routers (135,140, 145 and 150) are connected by a bi-directional link ‘not’ shown inthe figure. Nodes are inter-connected as shown in a ring topology usingtwo fibers 155, establishing bi-directional flow between them. Therouter may be or may include personal computer, mobile device, laptopand the device like. There is content server 105 attached to router 135wants to communicate with router 140, 145 and 150. The content servermay be or may include, for example, a mobile phone, a cellular phone, ahandheld device, a computing device, a computer, a mobile computer, aportable computer, a laptop computer, a handheld computer, a handhelddevice, a PDA device, a handheld PDA device, a mobile or portabledevice, or the like. According to some aspects, the content server 105may comprise a communication device connectable to one of the otherdevices/network via a wired or wireless connection.

Data traffic from the content server 105 flows to routers 140, 145 and150 via 135, and intermediate nodes 110, 115, 120, 125 and 130 throughthe communication channel 155. The above example not limited toPoint-to-Multipoint (P2MP) service, where the data traffic flows fromone node to multiple nodes. It may be Point-to-point service as well.After forming a topology, ERPS software is run on all the Nodes. In theERPS software there is a central node called RPL owner node which blocksone of the ports to ensure that there is no loop formed for the Ethernettraffic. The link which gets blocked by the RPL is called RingProtection Link (RPL). The other node which is connected to the RPL isknown as RPL-owner node. It uses control messages called Ring AutomaticProtection Switching (RAPS) message to coordinate the activities ofswitching on/off the RPL link.

In the figure link between node 110 and node 130 is defined as RPL, andNode 110 is a RPL-owner. Once this is defined, ERPS software will blockthe port on Node 110 that is facing the node 130. This is called asRPL-block. Once RPL-owner is designated, RPL-link is identified and RPLis blocked which leads to a bridge network with no loops for traffic inthe ring. Now data traffic from router 135 will flow into node 110. Node110 will learn the MAC source address of router 135 on the port facingtowards the same. The destination MAC address will not be presentbecause network was newly set up. So Node 110 will broadcast the trafficon all available ports. Clearly in a single ring topology node 110 willhave only one outgoing ring port. In multi ring topology there could bemultiple outgoing ring ports. Node 115 will receive traffic on portfacing node 110. Node 115 will learn the Source Address (SA) andbroadcast on port facing node 120. If the port facing router 140 is partof P2MP service then node 115 will forward the traffic on to the portfacing router 140, else it will not. However, it will continue toforward the traffic on the ERPS ring. This broadcast will continue tillnode 130. Finally, all routers will receive a copy of the P2MP service.ERPS protects traffic from failure of a link or node that form ring.Normally data traffic is sent over a Virtual Local Area Network (VLAN)ID that is different from ERPS control traffic. For example VLAN ID=1could be used as ERPS control traffic and VLAN ID=2, 3, 4 and 5 could beused as data traffic. Continuity Check message (CCMs) and other RingAutomatic Protection Switching (RAPS) control information are sent withVLAN ID=1. Since CCM, RAPS and Data traffic can potentially flow throughthe same fiber, or through same duct but different fiber, continuityfault indicated by absence of CCM will with high probability imply datapath fault. In other words, fault in CCM will be functionally linkedwith fault in all data VLAN IDs 2, 3, 4 and 5. That is, Fault in CCMwill imply fault in data traffic and not otherwise. The CCM includestrail of frames or packets.

FIG. 2 is a block diagram illustrating protection operation of ERPS whena fault has occurred between two of the network nodes in the system.Whether under fault or no fault scenario nodes will continue to exchangeCCMs with adjacent nodes. Failure is detected when a node fails toreceive CCMs from the adjacent nodes. Here in the system 200, thefailure in the link is between the nodes 215 and 220 i.e. the CCM sentby node 215 to 220 is not received and vice versa. Fault 260 between thenodes can also be detected using Loss of signal (LOS) mechanism. Oncethe fault 260 is detected between nodes 215 and 220 leads to trigger theERPS protection operation, where the ports of nodes 215 and 220 facingthe faulty path are blocked and nodes adjacent to fault 260 willbroadcast a signal failure (SF) message on the ring ports. In ERPS, SFis called as RAPS-SF meaning Ring Automatic Protection switching-SF,whenever a node receives RAPS-SF it will flush its Forwarding Data Base(FDB) entries and will forward the RAPS-SF onto the ring port. Now,momentarily the ring is broken into two parts or two linear trees. Onepath made between node 210 and 215, and the other path made of node 210,230, 225 and 220. When RPL-owner receives the RAPS-SF message, it willunblock the block on RPL. Now, all the nodes are connected through adifferent chain (Node 215-Node 210-Node 230-Node 225-Node 220). When allnodes have flushed their FDB (i.e. erasing all the learnt MAC addresses)then whenever a new frame arrives at Node 210 it will broadcast it onthe ring ports. Traffic to router 240 will flow through Node 210 andNode 215. Traffic to router 245 will flow through Node 210-Node 230-Node225-Node 220. Traffic to router 250 will flow through Node 210-node 230.The duplication of frame happens at Node 210, wherein one of the framesflows towards Node 215 and other frame flows towards node 230. At node230, duplication of frame happens again. One frame goes to router 250and other frame goes to Node 220 and finally to router 245. Protectionoperation is complete.

FIG. 3 is a block diagram illustrating restoration operation of ERPSwhen a fault has occurred between two of the network nodes in thesystem. Whether under fault or no fault scenario Nodes will exchangeContinuity Check Message (CCMs) with adjacent nodes. When link pathbetween Node 315 and 320 recovers, CCMs exchanged between them will bereceived at both nodes. Node 315 and 320 will wait for a period, guardtime, before sending RAPS-No Request message. This ensures that there isno intermittent fault. After expiry of guard time, Nodes 315 and 320will send RAPS-No Request message, RAPS-NR. When RPL-owner receivesRAPS-NR, the RPL-owner waits for a predetermined amount of time periodcalled Wait-To-Restore-Time (WTR Time), and when timer expires it willblock the RPL and send new RAPS message called RAPS-NR-RB (i.e. NoRequest, and Root has blocked the RPL). Momentarily, the chain is brokeninto two parts: Node 310-Node 315 and Node 330-Node 325-Node 320.Whenever a node receives RAPS-NR-RB message, nodes flush their FDBentries. If there is a port which has recovered from failure, then itwill be first unblocked and then the entries in the FDB will be flushed.So at nodes 315 and 320, blocks will be removed and then FDB will beflushed. Now there is only one chain that originally existed during thenormal operation of ERPS. Traffic from router 335 will follow the PathNode 310-Node 315-Node 320-Node 325-Node 330.

FIG. 4 is a functional diagram of two network nodes showing thepreferred protection mechanism, where ERPS terminates on IP/MPLS nodesaccording to an embodiment of the present invention. The system 400includes one or more nodes (410, 420, and 430), one or more routers (440and 450), and one or more medium for communication (460). Further, asshown in FIG. 4, one or more nodes and routers are connected in a ringnetwork. Particularly, the nodes 410, 420 and 430 are connected in anERPS mechanism and router 440 and 450 are IP/MPLS nodes. The nodes 410,420 and 430, and router 440 are connected through a communication medium(such as optical fiber, wireless, copper wire, cable etc.) for datatransfer.

In one embodiment of the invention, the nodes that are connected throughan ERPS mechanism terminate on one or more router. The invention is notrestricting to ERPS mechanism or ERPS state machine or ERPS technology,but method also applies to any type of deployment of ERPS network,single ring or multi ring which terminates on the IP/MPLS network. In anexample, referring FIG. 4 explains for single ring deployment, where thenode 410 terminates on router 440. To terminate a ring on IP/MPLSrouter, one breaks the ring into a chain and then connects the two endsof the chain to the IP/MPLS routers. When a ring (ERPS) terminates on anIP/MPLS router 440, two router ports will be used. The router portscould be on same physical router or on same logical router or onphysically separated routers or on logically separated routers. All thenodes and router are connected via bi-directional medium (for examplefiber, wireless, copper, cable, etc.).

In the below mentioned operation method, Continuity Check Message (CCM)and Loss of Signal (LOS) are used for fault detection between the nodesin the ring network. Bi-directional CCM, with unicast or multicast MACaddress specific to CCM, are used for fault detection between nodes 410and 430, via the router 440. Also, LOS is used for fault detectionbetween 410 and 440 and 430 and 440 as an example. Between bridges 410,420 and 430 bi-directional CCM or bi-directional LOS are used for faultdetection.

In a ring network where one or more nodes are connected to each other,each of the nodes transmits and receives CCM to check whether anycommunication failure has occurred within the network. If any of thenodes does not receive CCM from the other node, then there exists afault in that path. The different mode of operation has been explainedbelow under different condition of fault in different path duringcommunication.

Operation 1: Uni-Directional Loss of Signal (LOS) Faults Towards Router440 from Node 410

Referring to FIG. 4 (a), an example embodiment is shown when auni-directional Loss of signal (LOS) faults occurs towards router 440from node 410. In one mode of operation router 440 receives a Loss ofsignal (LOS) from a node 410, whenever there is a cut in thecommunication path between the router 440 and the node 410. The router440 and the node 430 are unable to receive continuity check message(CCM) from the node 410, since there is a failure in the communicationpath due to cut in the fiber towards router 440 from the node 410. Thenode 430 will not receive any LOS from router 440, since thebi-directional links between router 440 and the node 430 is healthy.Even though node 430 does not receive CCM from node 410, the node 430will not block its port towards the router 440. The node 430 will set abit i.e. Remote Defect Indication (RDI) indicating the fault in the CCMand transmits the same to node 410. Transmission of RDI in the CCM isvia router or tunneled through the router (i.e. tunneling through usingsome encapsulation method) or disjoint communication channel. Uponreceiving RDI at the node 410, the node 410 will block its port towardsrouter 440 and send signal fail (RAPS-SF) on both its ring ports.

Operation 2: Uni-Directional Loss of Signal (LOS) Fault Towards Node 410from Router 440

Referring to FIG. 4 (b), an example embodiment is shown when aUni-directional Loss of Signal (LOS) fault occurs towards node 410 fromrouter 440. In one mode of operation, node 410 receives a Loss of signalfrom the router 440, whenever there is a cut in the communication pathbetween the node 410 and the router 440. Based on the received LOS fromthe router 440, the node 410 will set a, RDI bit indicating the fault inthe Continuity Check Message (CCM) and transmits the same towards thenode 430 through router 440. The node 430 has not received any Loss ofSignal from the router 440, since the bi-directional links betweenrouter 440 and the node 430 is healthy. The node 430 received CCM withRDI bit set equal to zero from node 410. Upon receiving the CCM with theRDI bit set equal to zero at node 430 from node 410, the node 410 willblock the port towards router 440 and send Signal Fail (RAPS-SF) messageon both its ring ports.

Operation 3: Bi-Directional Fault Between Node 410 and Router 440

Referring to FIG. 4 (c), an example embodiment is shown when aBi-directional fault occurs between node 410 and router 440. In one modeof operation, node 410 receives a Loss of signal from the router 440 anddoes not receive Continuity check message (CCM) from node 430, wheneverthere is a cut in the communication path between the node 410 and therouter 440. Since the fault is a bi-directional fault, for the presentcase, the router 440 also receives Loss of signal from node 410. Thenode 430 does not receive any CCM from node 410 through router 440 buthas no Loss of signal from the router 440. The node 410 blocks it porttowards router 440 and sends signal fail notification on both its ports.

Operation 4: Two Uni-Directional Faults Between Node 410 and Router 440;and Router 440 and Node 430

Referring to FIG. 4 (d), an example embodiment is shown when twouni-directional faults occurs between node 410 and router 440; androuter 440 and node 430. In one mode of operation, node 410 receivesLoss of Signal (LOS) from the router 440. The router 440 receives LOSfrom node 430. The node 410 does not receive any Continuity CheckMessage (CCM) from node 430. Since, the node fails to receive CCM fromnode 430, the node 410 set RDI bit on the CCM and transmits the same tonode 430 through router 440. The node 430 receives CCM with theindication in the same that there is a fault at the receiving end. Thenode 430 is not aware of whether the fault is at router 440 or at thenode 410. Since, the node 430 has not received any LOS from router 440;node 430 will not block his ports. The node 410 blocks its port towardsrouter 440 and sends signal fail on both its ports.

FIG. 5 shows a 50 ms protection switching method according to anembodiment of the invention. At least some part of the method may beimplemented as a computer program stored on a computer-readable medium,such as firmware, for execution by a processor. Referring now to FIG. 5,there is illustrated a protection switching method 500 of a node havingcommunication failure in a ring, in a communication network, where thecommunication failure occurred on one or more non-ERPS device or node orin a communication channel.

At step 510, the method checks for loss of signal (LOS) at the non-ERPSdevice or on one or more of the node in a communication channel. The LOSsignal which is defined as per G.775 clause 4 is a received signal whichis less than stipulated amount of the actual signal. If the non-ERPSdevice has received LOS then there is a fault in the communicationchannel and the state jumps to step 550.

At step 520, the method checks periodically, say every 10 ms, forunicast or multicast Continuity Check Message (CCM) at both the node,where the CCMs are periodically transmitted from both the node throughone or more non-ERPS device. The CCM are transmitted and received by thenodes to check whether any fault is present in the communicationchannel. If the node fails to receive any LOS as well as CCM from theother node, then at step 530, the method set RDI=1 in the CCM subsequentto non receipt of LOS and CCM from the corresponding node, where theRDI=1 is a fault indication sent to the peer node.

At step 540, the method checks for value of RDI in the CCM subsequent tonon receipt of LOS and receipt of CCM from the corresponding node. Ifthe node has received LOS, then at step 550, the method set RDI=0 andsend the CCM to the corresponding node. Based on the receipt of faultcondition, the method at step 560 blocks the port of the node towardsthe non-ERPS device upon receipt of the fault notification andtransmitting signal fail notification message on both the ports in thering at step 570.

The main advantage of the correlation module or the above describedmethod is without any modification in the function of non-ERPS device,the protection switching method can be executed. This can beaccomplished by, whenever a failure has occurred in the link between thenon-ERPS device and the node, the port of the node that is connecting tothe non-ERPS device is blocked, where the blocking of ports of the nodeincludes, when the non-ERPS device is not complied with ERPS standard(G.8032). The non-ERPS device does not need to block ports. The systemwith the above described correlation module or method will work withoutor with no change within existing IP/MPLS nodes.

Although the flowchart 500 includes steps 510-570 that are arrangedserially in the exemplary embodiments, other embodiments of the subjectmatter may execute two or more steps in parallel, using multipleprocessors or a single processor organized as two or more virtualmachines or sub-processors. Moreover, still other embodiments mayimplement the steps as two or more specific interconnected hardwaremodules with related control and data signals communicated between andthrough the modules, or as portions of an application-specificintegrated circuit. Thus, the exemplary process flow diagrams areapplicable to software, firmware, and/or hardware implementations.

In the illustrated embodiment of FIG. 6, the protection switching methodand system for of a node having communication failure in a ring, in acommunication network, according to the present invention as discussedabove may be implemented as a series of software routines run bycomputer system 600 of FIG. 6. These software routines comprise aplurality or series of instructions to be executed by a processingsystem in a hardware system, such as processor 610 of FIG. 6. Initially,the series of instructions are stored on a data storage device 660,memory 620 or flash 630. It is to be appreciated that the series ofinstructions can be stored using any conventional computer-readable ormachine-accessible storage medium, such as a diskette, CD-ROM, magnetictape, DVD, ROM, etc. It is also to be appreciated that the series ofinstructions need not be stored locally, and could be stored on apropagated data signal received from a remote storage device, such as aserver on a network, via a network/communication interface 670. Theinstructions are copied from the storage device 660, such as massstorage, or from the propagated data signal into a memory 620 and thenaccessed and executed by processor 610.

In alternate embodiments, the present invention is implemented indiscrete hardware or firmware. For example, one or more applicationspecific integrated circuits (ASICs) could be programmed with theabove-described functions of the present invention.

Accordingly, a method and system is described in which a protectionswitching prevents loops in a layer-2 ring network. From the foregoingdescription, those skilled in the art will recognize that many othervariations of the present invention are possible. In particular, whilethe present invention has been described as being implemented in ametropolitan area network comprising multiple nodes or switches, itshould be noted that some of the logic described herein may bedistributed in other components of a network or implemented in a networkof different scope such as a local area network without departing fromthe scope of the present invention.

FIGS. 1-6 are merely representational and are not drawn to scale.Certain portions thereof may be exaggerated, while others may beminimized. FIGS. 1-6 illustrate various embodiments of the inventionthat can be understood and appropriately carried out by those ofordinary skill in the art.

In the foregoing detailed description of embodiments of the invention,various features are grouped together in a single embodiment for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimedembodiments of the invention require more features than are expresslyrecited in each claim. Rather, as the following claims reflect,inventive subject matter lies in less than all features of a singledisclosed embodiment. Thus, the following claims are hereby incorporatedinto the detailed description of embodiments of the invention, with eachclaim standing on its own as a separate embodiment.

It is understood that the above description is intended to beillustrative, and not restrictive. It is intended to cover allalternatives, modifications and equivalents as may be included withinthe spirit and scope of the invention as defined in the appended claims.Many other embodiments will be apparent to those of skill in the artupon reviewing the above description. The scope of the invention should,therefore, be determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled. Inthe appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein,” respectively.

We claim:
 1. A protection switching method of a first node havingcommunication failure in a ring network, the communication failurehaving occurred on at least one of a non-Ethernet Ring ProtectionSwitching (non-ERPS) device, a plurality of nodes, and a communicationchannel, the method comprising: checking for receipt of a Loss of Signal(LOS) message from at least one of the non-ERPS device and the pluralityof nodes in the communication channel of the ring network, the LOSmessage indicating that a received signal level is lower than astipulated amount; checking periodically for receipt of Continuity CheckMessages (CCMs), the CCMs being separate from the LOS message andperiodically transmitted and received among the plurality of nodesthrough at least one non-ERPS device, transparently; appropriatelysetting a Remote Defect Indication (RDI) bit in CCMs generated from thefirst node and transmitting the CCMs through the non-ERPS device or overa disjoint communication channel, the RDI bit being set based on atleast one of receipt of the LOS message or non-receipt of the LOSmessage, and the non-ERPS device being not capable of setting the RDIbit in CCMs; checking for a fault condition based on the received CCMsand receipt or non-receipt of the LOS message; and in the faultcondition, blocking a port of the first node connected to the non-ERPSdevice and transmitting a signal fail notification message on ports ofthe first node connected to the ring.
 2. The protection switching methodas claimed in claim 1, further comprising: reverting traffic to a normalstate upon recovery from the fault condition using a guard time andwait-to-restore timer.
 3. The protection switching method as claimed inclaim 1, comprising setting the RDI=0 in CCMs subsequent to non-receiptof the LOS message and receipt of CCMs from a correspondingbridge/switch.
 4. The protection switching method as claimed in claim 1,comprising setting the RDI=1 in CCMs subsequent to non-receipt of theLOS message and CCMs from a corresponding node, the RDI=1 being anindication sent to a peer node that an error in CCM frames has beendetected.
 5. The protection switching method as claimed in claim 1,wherein the port of the first node connected to the non-ERPS device isblocked when the non-ERPS device is not in compliance with ERPS standard(G.8032).
 6. The protection switching method as claimed in claim 1,wherein a communication channel between the node and the non-ERPS deviceis a uni-directional link or a bi-directional link, and the non-ERPSdevice is at least one selected from the group consisting of: anInternet Protocol/Multiprotocol Label Switching (IP/MPLS) router, anAsynchronous Transfer Mode (ATM) switch, a Frame Relay, an EthernetSwitch, and a Resilient Packet Ring (RPR) switch.
 7. The protectionswitching method as claimed in claim 1, wherein the CCM includes a trailof frames or packets and the CCMs are transceived (IEEE 802.1ag) betweenthe plurality of nodes without any modification in the function of thenon-ERPS device.
 8. A protection switching system of a first node havingcommunication failure in a ring network, the communication failureoccurring on at least one of: a non-Ethernet Ring Protection Switching(ERPS) device, a plurality of nodes, and a communication channel, thesystem comprising: a plurality of nodes interconnected in a ringtopology forming the ring network, including a first node; at least onenon-ERPS device within the ring, the non-ERPS device being not capableof setting a Remote Defect Indicator (RDI) bit in Continuity CheckMessages (CCMs); and a network channel connected to each of theplurality of nodes via at least one of the non-ERPS device fortransmitting and receiving CCMs, wherein the first node is configuredto: determine a failure condition within the communication channel by:checking for receipt of a Loss of Signal (LOS) message from at least oneof the non-ERPS device and the plurality of nodes in the communicationchannel of the ring network, the LOS message indicating that a receivedsignal level is lower than a stipulated amount, checking periodicallyfor receipt of CCMs, the CCMs being separate from the LOS message andperiodically transmitted and received among the plurality of nodesthrough at least one non-ERPS device, transparently, appropriately setthe RDI bit in its CCMs and transmit the same to each of the otherplurality of nodes through the non-ERPS device or through a disjointcommunication channel, in response to the failure condition, and block aport of the first node connected to the non-ERPS device in the failurecondition, and transmit a signal fail notification message on ports ofthe first node connected to the ring network, in response to the failurecondition.
 9. The protection switching system as claimed in claim 8,wherein the first node is further configured to: set the RDI=0 in CCMssubsequent to non-receipt of the LOS message and receipt of CCMs from acorresponding node, and set the RDI=1 in CCMs subsequent to non-receiptof the LOS message and CCMs from a corresponding node, the RDI=1 beingan indication sent to a peer node that an error in CCM frames has beendetected.